1. Field of the Invention
The present invention relates to a thin film transistor device having a matrix circuit for applying an electric field to a liquid crystal display device and a driving circuit for driving the matrix circuit.
2. Description of the Related Art
For example, there is used an active matrix type liquid crystal display comprising a matrix circuit for applying an electric field to a liquid crystal display device and a peripheral driving circuit for driving the matrix circuit, each circuit being formed of field effect type thin film transistors.
FIG. 4 shows one example of the circuit structure of such a conventional active matrix type liquid crystal display.
This active matrix type liquid crystal display comprises a matrix circuit 1, one peripheral driving circuit 2 for supplying a scanning signal to an address bus of the matrix circuit 1, and other peripheral driving circuit 3 for supplying a display signal to a data bus of the matrix circuit 1. In the matrix circuit 1, there are formed a large number of scanning electrodes 4 arranged in row and a large number of display electrodes 5 arranged in column, and a thin film transistor 7 for the matrix circuit is formed for each pixel 6 (liquid crystal) corresponding to each cross point of the scanning electrodes 4 and the display electrodes 5. A gate electrode of each thin film transistor 7 is connected to each scanning electrode 4, and a source electrode is connected to the display electrode 5. A drain electrode of each thin film transistor 7 is connected to a pixel capacitive element 8 having a transparent electrode connected to each pixel in parallel. One peripheral circuit 2 comprises a thin film transistor (not shown) for one peripheral circuit, connected to one end of the scanning electrode 4. Other peripheral circuit 3 comprises a thin film transistor for other peripheral circuit, connected to one end of the scanning electrode 5. Then, if the thin film transistor 7 for matrix circuit is turned on, display data is written to the capacitive element 8 in the form of an electric charge. If the thin film transistor 7 for matrix circuit is turned off, the pixel 6 is driven in a predetermined time by the written electric charge.
According to such a conventional thin film transistor device, the thin film transistor 7 forming the matrix circuit 1 and the thin film transistor forming the peripheral driving circuit were formed in the same structure. Therefore, characteristics of the transistor such as a switching speed of the thin film transistor, a cut-off speed or the like were the same in both transistors.
In recent years, display with extremely high precision has been required in the active matrix type liquid crystal display, and it has been needed that the number of thin film transistors which form the matrix circuit 1 and the peripheral driving circuits 2 and 3, is increased.
However, as the number of the thin film transistors is increased, the consumption current to be consumed in the entire device is increased. In order to solve this problem, the cut-off current of each thin film transistor 7 must be controlled to be small. On the other hand, the switching speed of each thin film transistor must be increased in accordance with increase in the number of the thin film transistors. However, as is well-known, the on-current must be increased in order to increase the switching speed, and the cut-off current is also increased by the increase in the on-current. For this reason, the above two requirements cannot be satisfied.